Selective radio frequency control system



Sept. 30, 1947. s. w. SEI-:LEY

SELCTIVE RADIO FREQUENCY CONTROL SYSTEM Filed Nov. 6, 1943 fidi/70 INVENTOR. w w

BY W

Patented Sept. 30, 1947 SELECTIVE RADIO FREQUENCY CONTROL SYSTEM Stuart W. Seeley, Roslyn Heights, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application November 6, 1943, Serial No. 509,232

(Cl. 25o-2) 9 claims. 1

The invention covered herein may be manufactured and used by or for the Government of the United States for all governmental purposes without payment to me, or my assigns, of any royalty thereon.

My present invention generally relates to remote control by modulated high frequency waves of a plurality of independent devices, and more particularly to a radio remote control system capable of secretly controlling from a single remote point a selected one oi a large number of operations.

One of the main objects of my present invention is to provide a radio remote control system which is secret to a high degree, and which controls a. plurality of independent receivers free from radiation or spurious operation from any source.

Another important object of my invention is to provide equipment intended selectively to detonate land mines by means of radio signalling.

Another important object of my invention is to provide a transmitter unit which replaces the exciter in a standard type of radio transmitter and supplies a constant-amplitude, frequency modulated signal to the transmitter, while a. receiver unit is designed to be located at or near a land mine and has provisions for detonating an associated land mine.

Another object of my invention is to provide a remote control system wherein a large number of separate receivers may be operated from a single transmitter, or considering a single receiver the chance of a random signal operating that receiver is very slight.

A further object of my invention is to provide radio control equipment having a high degree of secrecy, accomplished by the type of modulation transmitted and by providing a receiver that functions only on that particular type of modulation.

A more specic object of my invention is to provide a radio-controlled land mine receiver which is designed to be free from radiation, spurious operation by either the enemy or static, and blocking by the enemy,

I attain the above objects by a new and improved method of, and means for, generating at the control point or points wave energy in which a plurality of series of distinct and separate incidents, such as changes in timing, are caused to occur. The frequency of the generated wave energy on which the series of incidents occur is changeable, and thus a large number of incident combinations are available. The incident combinations may be used at a receiving point or points to perform a selected function, so that an operator at the transmitter by controlling the transmitted wave may select and control one of a large number of operations at a selected receiver, or a large number of separated receivers or selected receivers of groups of receivers; or any selected one of a large number of receivers performing a single operation may be controlled.

In a particular application of my new method a selected sub-carrier is modulated by a selected frequency, and is used to modulate a selected carrier to provide many different combinations. These transmitted combinations then cooperate with receiver relays wherein many different combinations are provided so that a great number of combinations are possible with the elements provided. Thus, each of a great many separate receivers can be operated separately on its own particular individual pre-set combination.

The general military objective 0f my invention is that a few, or a few hundred, receivers equipped with detonating mechansims may be remotely fired separately or in groups (depending on the coding of the ring combination). The high order of numbers of combinations provided is to minimize the chance of inadvertent operation of a particular receiver from noise, interference or even operation of the transmitter to detonate mines in another area. or category (for example, docks, airelds, power stations or barracks may be operated selectively) Thus, if millions of elements must enter into a proper combination to secure a, detonation of a particular receiver or group of receivers, the chances of inadvertent or enemy operation becomes vanishingly small. The receiver relays may each complete a circuit to control a desired operation. The number of tones, sub-carriers, carriers, and receiver relay combinations may be increased to expand the total number of combinations to a very large number. In the particular application described, the receiver relays complete a squibb circuit to detonate land mines or similar explosive charges. Obviously, the means of my invention may be put to other purposes,

The wave energy modulated as described above, which is of constant amplitude, is in this particular application used to excite a frequency modulated type of radio transmitter. The controlled receivers are to be located at or near a land mine, and the relays therein complete circuits for detonating the associated land mine. This system is quite flexible in that the number of combinations may be increased or decreased by minor changes in the receiver units.

One of the factors of prime importance in this application is the reception method employed in the system. The receiver of the system is so constituted that it will provide amplification at the second harmonic of a predetermined `modulation frequency of an incoming carrier. The second harmonic is produced by simple narrow band tuned circuits at the carrier frequency. The detector of a conventional frequency modulation receiver is usually preceded by amplification of a broad band of frequencies at the carrier frequency followed by a discriminator to produce amplitude modulated carrier energy. In my receiver amplification and discrimination are accomplished with the use of sharply tuned circuits which produce second harmonic effects. The generic expression angle modulation is employed herein to include frequency modulation, phase modulation or hybrids of the latter two forms of modulation.

A system as disclosed herein is secret to a high degree. This is accomplished by frequency modulation of a sub-carrier by a desired tone and frequency modulation of a carrier by the frequency modulated sub-carrier, and providing receivers responsive only to such modulation. Moreover, since the tone selected is at the discretion of the transmitter operator as guided by selection preknowledge) of the receiver fto be the control medium, as is the sub-carrier and carrier frequency selected, the possibility that an enemy transmitter will be operated even by chance in the proper manner to take control through a receiver is practically nil. Moreover, by providing receivers which function only on receipt of the particular type of transmitted modulation, there is stillV less probability that an enemy transmitter could operate the receivers even by change to explode a mine.

Still other features and objects of my invention will best be understood by reference to the following description, taken in connection with the drawing, in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect.

In the drawing:

Fig. 1 is a. schematic representation of a transmitter unit employed in the present system,

Fig. 2 illustrates in schematic manner a receiver utilized in the control system,

Fig. 3 graphically .shows discrimination characteristics employed at the receiver.

In my present application there is disclosed and claimed the general control system which consists of the transmitter unit of Fig. 1 and the receiver of Fig. 2. A specific form of the transmitter unit has been disclosed and claimed by Harmon B. Deal, John A. Rankin and William R. Alexander in U. S. application Serial No. 497,006, led August 2, 1943. A specific embodiment of the receiver is disclosed and claimed by said Deal et al., in U. S. application Serial No. 514,964, led December 20, 1943.

Considering, now, the networks employed by me at the transmitter, there is provided a multiple frequency modulated carrier wave. The wave is produced by frequency modulating a carrier wave of a predetermined high frequency by a. modulating current which is itself a frequency modulated sub-carrier wave. The source of the modulation signals of the sub-carrier wave is designated by numeral l in Fig, 1. This source may be any form of modulation frequency oscillator dfi lil"

adapted to be varied in frequency over a fairly wide frequency range. The expression is used hereinafter to designate the term "frequency modulation."

If the oscillator I is an audio frequency oscillator, it can include a frequency selector mechanism 2 adapted to select aidesired one of a plurality of audio frequencies lying between 200 and 1200 cycles. The modulation source need not be in the audio range. Superaudible or subaudible currents can be used as the modulating currents, where desired. The sub-carrier wave source is indicated by numeral 3. It may be an oscillator adapted. to generate sub-carrier frequencies in a band of G0 to 140 kilocycles (kc). The frequency range is chosen so that adequate frequency modulation of a selected sub-carrier oscillation may take place.

The frequency modulation is performed by means of any well known form of electronic reactance modulator device. As shown, numeral 4 denotes an FM modulator device which 'has its input terminals coupled to the secondary winding of transformer 5. The primary winding is included in circuit with the output terminals of modulation source I. The mechanical interrupter, or switch, B can be of any Well known construction. For example, switch E may be part of a manual dialing mechanism. Since the manual dial 6, such as a telephone dial, is in circuit with the output leads of oscillator l the modulation oscillations are pulsed at a repetition rate of a predetermined number of times per second, which permits the transmission of from one to the predetermined number of pulses (say l0) per dial operation.

The modulator 4 can be of any type used in the art of FM transmission. For example, the plate to cathode impedance `of an electron discharge tube could be connected across the tank circuit l of sub-carrier oscillator 3. VSuch impedance could be made to vary in magnitude and direction by applying to the grid of the reactance tube an alternating Voltage in phase Af luadrature with the plate voltage. The output of transformer 5 would, in that case, be applied to the reactance tube grid to vary the reactive effect of the plate to cathode impedance. Such an electronic reactance has been disclosed by Charles Travis in his U. S. application Serial No. 19,563, filed January 28, 1936.

Of course, the control grid to cathode impedance of a tube could be used, if desired, and the modulation voltage applied to the grid. In this case, also, the grid to cathode impedance of the modulator tube would be connected across the tank circuit l. The frequency selector device 8, which may be by way of illustration a variable condenser, may be adjusted to tune tank circuit l to the particular sub-carrier frequency desired. Those skilled in the art of FM transmission are well aware of the theoretical basis for frequency modulating a wave by modulation signals. Briefly, the frequency of the sub-carrier wave will be deviated to an extent dependent upon the amplitude of the modulating signal, while the frequency of the modulating signal determines the number of times per second that the change or deviation in frequency of the sub-carrier takes place. It will be noted that the selected modulation oscillation will have a sinusoidal character, while the sub-carrier amplitude is presumed to be uniform. These characteristics, however, are not restrictive.

The FM sub-carrier wave is vnew employed to frequency modulate the carrier oscillations whose frequency is determined by the selector device 9 of tank circuit IIl of carrier oscillator II. By Way of specific example, let it be assumed that oscillator II can cover a range of 5 to 10 megacycles (rnc). The FM modulator I2 may be constructed in the same manner as modulator 4. The FM sub-carrier energy is applied, as at I3, to the modulator I2. If the latter comprises a reactance tube, as was described for modulator 4, the FM sub-carrier energy would be applied to the grid of the reactance tube. The plate to cathode impedance would then be connected across tank circuit I0. The frequency deviation of the selected wave of oscillator II would occur as previously described. The extent of frequency deviation of the carrier frequency would depend on the instantaneous amplitude of the FM sub-carrier energy, While the instantaneous frequency of the latter would determine the number of times per second that the carrier frequency deviation would take place relative to the mean carrier frequency. The product, or output, of the oscillator I I would be a multiple frequency modulated Wave, or more specifically a double frequency modulated carrier wave. The latter wave energy may then supply the modulated excitation for a frequency multiplier transmitter operating through the to 40 mc. band. Of course, the multiplier may be dispensed with if the deviation of the carrier is adequate, or if the receiver operates Within a relatively small deviation range.

To demonstrate the secrecy aspect of the system, let it be assumed that there are eight audio frequencies alternatively used at the modulation source I; ve frequencies are available at the sub-carrier source 3; and five frequencies are also available at the carrier oscillator II. This provides at once a total of 200 combinations. Now, if the dial mechanism Ii provides ten interruptions, there are many more combinations possible. There are additional factors of secrecy that cannot be evaluated numerically. It would be difficult, if not impossible, to determine what type of modulation was being transmitted by listening to the transmitted signal. This would be true even after knowing the exact type of modulation, since it would be time-consuming to produce the Wave for study in the laboratory. The transmitter and antenna could be air-borne and thereby permit the use of a low power transmitter.

The receiver unit for detecting and utilizing the double FM carrier wave is shown in Fig. 2. 'The receiver utilizes a novel method of operation in which no local oscillator, or other radiatable signal, is necessary in order to derive the benefits of selectivity and sensitivity inherent in a superheterodyne receiver. No radiation of continuous nature is possible from the receiver. From a general viewpoint, the reception of the transmitted wave is accomplished in the following manner:

The received FM signal energy is amplified by a sharply tuned radio frequency amplifier which is resonant to the predetermined mean carrier frequency. Since the radio frequency signal energy is frequency modulated, the carrier may be thought of as varying in frequency above and below its normal or quiescent frequency once during each cycle of the modulating frequency. Of course, the modulating frequency is the modulated sub-carrier. This process of modulation sweeps the carrier frequency back and forth across the single-peaked resonance characteristic of the radio frequency amplifier thereby imparting amplitude modulation to the carrier. For each cycle of frequency modulation of the carrier the resulting amplitude modulation has two maxima and two minima.

Hence, the amplitude modulation is double the original frequency of the sub-carrier modulation current. The amplitude modulated carrier Wave energy is now subjected to detection. The detected energy is frequency modulated wave energy whose mean frequency is double the su -carrier frequency and Whose modulation is made up of components provided originally at oscillator I, i. e., the audio oscillations. I term such frequency modulated Wave energy of double the sub-carrier frequency intermediate frequency energy (I. F. for brevity). The term should not be understood as implying the use of a local oscillator, as is done in superheterodyne receiver practice. The expression "I, F." is used herein, because the double sub-carrier frequency is intermediate the received carrier frequency and the audio modulation frequency.

The I. F. energy is amplified by an amplifier sharply tuned to the second harmonic of the sub-carrier frequency. During this process of sharply selective amplilication the I, F. energy is amplified, and has imparted to it double frequency amplitude modulation in a manner identical to that described above in connection with the selective amplification of the FM carrier wave energy. Subsequent to the selective I. F. amplication the resulting energy is subjected to detection. The amplitude modulated wave energy in this case has a mean frequency which is the double subcarrier frequency, but its modulation frequency is double the original audio modulation frequency. In other words during this step of deriving amplitude modulated energy from the FM Wave energy the doubling of the modulation frequency occurs, exactly in the same manner as was explained in connection with the conversion of the FM carrier wave energy into amplitude modulated wave energy. The detected double modulation frequency current is amplified in a tuned audio frequency amplifier. Rectification of the audio frequency current follows, and the resulting direct current is employed to operate a relay mechanism. The relay mechanism may be constructed in any desired fashion to operate the controlled device, and the latter in the specific case of this application would be the land mine detonator.

Considering the receiver unit more specifically, the numeral I4 designates any desired type of signal collector. Depending upon the installation of the receiver unit, the collector I4 could be a grounded antenna circuit, a dipole or any other known pickup device capable of collecting wave energy whose carrier frequency is in the operating carrier range of 20-40 mc. The carrier amplifier I5 may be of any well known construction. Its resonant input circuit IE consists of the customary parallel resonant circuit. The variable condenser I1 may be adjusted to tune the circuit I6 to any of the carrier frequencies in the 20-40 mc. range. The constants of circuit IB are chosen so as to have a high degree of selectivity. In other words, the circuit is designed to have a low damping.

In Fig. 3 there is shown the sharply peaked resonance curve which it is desired to have for the input circuit I6 of amplifier I5. It will be noted that the curve A in Fig. 3 is very sharp. The peak frequency of the curve will, of course, depend upon the tuning of the carrier amplifier input circuit. The "Q of the circuit is chosen accolse# I so as to be quite high. If desired, aV plurality of carrier amplifier circuits, each with its respective sharply tuned input circuit, may beA arranged in cascade.

The detector I8 may be of any desired type. For example, it may be a simple diode. In any case its input circuit I9: will be sharply tuned to the selected carrier frequency, that is, the resonance curve of circuit I9 will be approximately the same as that of circuit I6. The effect of the highly selective input circuits If6` and I3 on the FM carrier wave is represented in Fig. 3'.

The curve B in Fig. 3 illustrates the nature of the change which occurs in the applied modulated carrier wave energy due to the selective input circuits IB and I9, it being understood that the tuned circuit I6 and amplifier I5 do 'not change the frequency of the received wave. The curve B is derived by considering the responses at the various points along curve A when a sinusoidal current is applied to circuits I6 and I9. Curve C represents the sinusoidal frequency deviation of the frequency modulated carrier Wave applied to circuit IB. The various points I, 2, 3, 4, 5 on curve C correspond in time with the similarly numbered points on curve B, and the vertical time scale of curve C is the same as the horizontal time scale of curve B. It will be recalled that the signal energy picked up by antenna I4 is an FM Wave of constant amplitude. Such an FM wave is sinusoidal in character between any pair of successive peaks. It is readily seen that there Will be developed across circuits I6 and I9 voltages in accordance with the amplitude modulated carrier curve B due to their response characteristics having the shape represented by curve A. However, the modulation frequency of such an amplitude modulated wave is the second harmonic, or double the value, of the sub-carrier frequency which actually is the modulation on the carrier picked up at signal collector III,

The detector I8 acts to rectify the amplitude modulated carrier Wave energy. The detected output is applied to the highly selective tuned input circuit 211 of I. F. amplifier 2|. It will be understood that circuit 20 is sharply tuned to the frequency of the wave delivered by detector I8 which is double the sub-carrier frequency at the transmitter, and is frequency modulated in accordance with the energy being produced by the source I at the transmitter. The sharp selectivity characteristic of circuit 20 acts in the same manner as described in connection with Fig. 3. In other words, the intermediate frequency FM Wave energy applied to circuit 20 is translated into amplitude modulated wave energy whose mean frequency is the second harmonic of the sub-carrier frequency, but Whose modulation frequency is 4double that of the energy produced by the source I. This amplitude modulation of the intermediate frequency output of detector I8 is increased by the action of the highly selective input circuit 22 of the detector 23. Consequently, there is applied to detector 23 an amplitude modulated Wave whose modulation component is the second harmonic of the selected modulation frequency which was applied by oscillator I in Fig. 1 to the sub-carrier oscillator 3. The detector 23 may be of any well known type, as for example of the same characteristics as detector I8.

The modulation voltage developed at the output terminals of detector 23 is applied to the audio frequency ampli-fier 24. This amplifier will include at least one tuned circuit. The tuned circuit will be resonant to, and have impressed thereupon wave energy of, a frequency which is the second harmonic of the modulation frequency selected at oscillator I. Those skilled in the art are fully acquainted with the manner of constructing an audio-tuned. circuit. Of course, one or more separate` stages of tuned audio frequency amplification may Ibe employed at amplier 24; After amplification the rectifier 25 rectiiies the wave energy impressed upon it. The direct current output of rectifier 215 is then utilized to operate a relay 26.

Generally speaking the relay 26. may be any form of ratchet relay capable of being actuated Iby the dialing impulses. The numeral 21 designates a controlled device. By way of specific illustration, this device 21 may be the detonating circuit of a land mine. Thev detonating circuit may be arranged tobe closed upon a predetermined motion, or Steppin of relay 26. Since the specific construction of the relay 26 and controlled device 21 is not a. material part of my present invention, it is not believed necessary to show the details thereof. Attention is directed to the specific construction disclosed and claimed in the aforesaid Deal` et al. application for a particular illustration of such arelay.

The probability of operating the receiver unit from any source other than the transmitter unit is negligible. The unique nature of the multiple `frequency modulated carrier Wave signal transmitted, and the rather short periods it is on the air, makes an analysis of its real nature difficult to ascertain even for a well equipped laboratory, and well beyond the capabilities of field equipment. Until samples of the equipment fall into enemy hands and are referred to their laboratories for analysis, the method itself lends itself to secrecy. Assuming that the enemy had become familiar with the method employed, and moreover had in some way discovered the frequency band used for the carrier, sub-carrier and modulation frequencies, he would still have to set up correct combinations of these variables before dialing would become possible. The very large number of combinations would prove practically impossible to solve.

Reviewing the operation of the system, it will be understood that each receiver unit will be adjusted so as to respond to a particular transmitted wave representing a predetermined combination of frequencies. For specific illustration let it be assumed that a gilven controlled device 2.1 can be actuated only upon energization. of relay 26 a total of four pulses. 'This means at once that at the transmitter unit the dialing mechanism 6 will have to be closed four times in order to secure any response at the controlled device 21. Let it also be assumed that the selected carrier frequency is 5Y mc.; the selected sub-carrier frequency is 60 kc.; and the selected modulation frequency atoscillator I is 200 cycles. The selecting mechanism of the respective oscillators at the transmitter unit will 4be adjusted so as to provide the aforesaid frequencies at each oscillator network II, 3 and I. The resultant radiated wave will then be a double frequency modulated Wave Whose carrier frequency is 20 mc., whose sub-carrier frequency is 60 kc., while the modulation on the sub-carrier is 20.0 cycles. The Wave form will :be sinusoidal', since each oscillator has a constant amplitude and operates at a fixed frequency. My invention contemplates repeated and' multiple frequency modulation of a carrier by one or more modulated sub-carriers.

aliases? The particular receiver unit which is to respond to the aforementioned double frequency modulated wave will have its tuned circuits IE and I9 sharply tuned to the carrier frequency of 20 mc. The tuned circuits 20 and 22 will have the resonant frequencies thereof adjusted to 120 kc., since that `value is the second harmonic of the sub-carrier frequency. The tuned audio amplifier 24 Will be adjusted to respond to 400 cycles.

Hence, it will be seen that relay 25 cannot be energized in any event unless each of the successive sharply tuned circuits of the receiver unit i has applied to it energy of the particular frequency to which each selector circuit is tuned. Upon development of 400 cycle voltage at the input terminals of rectifier 25, relay 2t will be energized. Finally, upon energization of the relay 26 a number of timos corresponding to the dialing at 6, the controlled `device 2l will he actuated. There is no source of radiation in the receiver unit, since no local oscillator is employed. High selectivity is secured lby virtue of the plurality of cascaded highly selective circuits employed between the antenna I4 and detector 25.

The possibility of inadvertent operation of one receiver unit when other adjacent receivers in the same mine field are operated, is of the same order of magnitude as that to enemy action. Since transmission for radar, aircraft, portable equipment and other radio communication do not use the same system of modulation disclosed here, there is a negligible possibility of such transmission causing misre, even though they might operate in the same carrier band.

While I have indicated and described several systems for carrying my invention into eiiect, it will loe apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of my invention.

What I claim is:

1. In a method of radio communication wherein transmission is accomplished by the steps of frequency modulating a sub-carrier current with a predetermined modulation current, frequency modulating a. carrier of high frequency with the frequency modulated sub-carrier current and transmitting the resulting frequency modulated carrier wave, the improvement which comprises subjecting the latter wave to relatively high selective action thereby to derive from the wave amplitude modulated carrier wave energy whose modulation frequency is at the sub-carrier second harmonic frequency, detecting the amplitude modulated carrier Wave to produce a frequency modulated wave whose mean frequency is said sub-carrier second harmonic, subjecting the latter frequency modulated wave to successive high selective action and detection to derive therefrom the second harmonic of the original modulation, and utilizing the last named second harmonic.

2. In a radio system adapted to utilize a multiple frequency modulated carrier wave whose mean carrier frequency is deviated at a subcarrier frequency which is in turn deviated at a predetermined modulation frequency; the method of reception which includes deriving from the multiple modulated carrier wave a modulated wave whose mean frequency is the sub-carrier second harmonic deviated at said predetermined modulation frequency, deriving energy from said last modulated wave whose frequency is the second harmonic of said predetermined modulation frequency, and utilizing the second harmonic modulation energy.

3. In a receiver system, a plurality of cascaded resonant circuits of high selectivity, at least one detector interposed between a pair of said cascaded circuits, means for tuning the resonant circuit following the detector to the second harmonic of the modulation frequency of a received modulated carrier Wave, and the resonant circuit preceding said detector being tuned to the mean frequency of said received carrier wave.

4. In a receiver of angle modulated carrier waves, a first selector circuit tuned to the mean frequency of said waves, said selector circuit having sufficient selectivity to provide at its output terminals amplitude modulated carrier Wave energy of said mean frequency but Whose modulation frequency is the second harmonic of the original modulation frequency, and means for detecting the amplitude modulated wave energy.

5. A secrecy method of controlling a remote device by radio waves, which comprises radiating a double frequency modulated carrier wave made up of a selected combination of carrier, subcarrier and modulation frequencies, subjecting the radiated wave to successive highly selective amplification and detection at frequencies equal to respectively the carrier. sub-carrier second harmonic and modulation second harmonic frequencies, and controlling the said device in response to the modulation second harmonic energy.

6. In a receiver of angle modulated carrier waves, a first selector and amplifier circuit tuned to the mean frequency of said waves, said selector circuit having sufiicient selectivity to provide at its output terminals amplitude modulated carrier wave energy of said mean frequency but whose modulation frequency is the second harmonic of the original modulation frequency, means for detecting the amplitude modulated wave energy, and further means to demodulate the output of said detecting means.

'7. A method which comprises radiating a double frequency modulated carrier wave made up of a selected combination of carrier, subcarrier and modulation frequencies, and subjecting the radiated wave to successive highly selective amplification and detection at frequencies equal to respectively the carrier, subcarrier second harmonic and modulation second harmonic frequencies.

8. A method of secret radio communication which comprises the combination of frequency modulating a first carrier with a modulation current, frequency modulating a second carrier of high frequency with the frequency modulated carrier, transmitting the resultant frequency modulated carrier wave, and the improvement comprising the steps of deriving from the latter wave amplitude modulated carrier Wave energy whose modulation frequency is at the second harmonic of said rst carrier frequency, detecting the amplitude modulated carrier wave to produce a frequency modulated wave whose frequency is said second harmonic, subjecting the latter frequency modulated wave to successive high selective action and detection to derive therefrom the second harmonie of the original modulation, and utilizing the last named second harmonic.

9. In a radio system utilizing a radiated wave comprising a multiple frequency modulated carrier wave composed of a first carrier frequency 2,428,297 11 12 modulated by a. frequency modulated second car- TES TEN rier, the improvement comprising deriving from UNITED STA PA TS the received wave a frequency modulated wave Number Name Date whose mean frequency is the second harmonic 2,165,847 Gerhard July 11. 1939 of said second carrier frequency, and deriving 5 230,243 Ha'cke Feb- 4, 1941 from the latter Wave modulation signals. 1.917395 P01111 July 11 1933 2,063,354 Thorp Dec. 8, 1936 STUART W, SEELEY, 1,361,522 Espenschied Dec. 7, 1920 1,565,521 Stone et al. DEC. 15, 1925 REFERENCES CITED 10 2,085,008 Crosby June 29, 1937 2,233,183 Rodel' Feb. 25, 1941 The following references are of record in the 2,298,409 Peterson Oct. 13, 1942 file of this patent: 1,662,877 Almquist Mar, 20, 1928 

